Add basic Flakes support

Co-authored-by: Alex Zero <joseph@marsden.space>

.github/workflows/build.yml

@@ -7,7 +7,13 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2.3.4
-      - uses: cachix/install-nix-action@v12
+      - uses: cachix/install-nix-action@v13
+        with:
+          install_url: https://github.com/numtide/nix-unstable-installer/releases/download/nix-2.4pre20210604_8e6ee1b/install
+          install_options: '--tarball-url-prefix https://nixos-nix-install-tests.cachix.org/serve'
+          extra_nix_config: |
+            experimental-features = nix-command flakes
+
       - run: nix-build
   tests:
     runs-on: ubuntu-latest